Transmitter and receiver synchronization for wireless telemetry systems

ABSTRACT

An acoustic modem for communication in a network of acoustic modems via a communication channel. The acoustic modem comprises a transceiver assembly, transceiver electronics, and a power supply. The transceiver assembly is adapted to convert acoustic messages into electrical signals. The transceiver electronics is provided with transmitter electronics and receiver electronics. The transmitter electronics cause the transceiver assembly to send acoustic signals into the communication channel. The receiver electronics comprises at least one microcontroller adapted to execute instructions to (1) enable the receiver electronics to receive electrical signals indicative of the acoustic message from at least one other acoustic modem via the transceiver assembly, (2) estimate a carrier frequency of the electrical signals by analyzing an estimation frame of the electrical signals, (3) estimate a starting time of a data frame of the electrical signals by synchronizing with a synchronization frame of the acoustic message in parallel with at least two bit rates, and (4) decode the data frame. The power supply supplies power to the transceiver assembly and the transceiver electronics.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

TECHNICAL FIELD

The present invention relates to telemetry systems for use with installations in oil and gas wells or the like. In particular, the present invention relates to the transmission of data and control signals between a location down a borehole and the surface, or between downhole locations themselves.

BACKGROUND ART

One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between downhole locations themselves. For example, in the oil and gas industry it is desirable to communicate data generated downhole to the surface during operations such as drilling, perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to downhole tools or instruments to effect, control or modify operations or parameters.

Accurate and reliable downhole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded digital signals.

Downhole testing is traditionally performed in a “blind fashion”: downhole tools and sensors are deployed in a well at the end of a tubing string for several days or weeks after which they are retrieved at surface. During the downhole testing operations, the sensors may record measurements that will be used for interpretation once retrieved at surface. It is only after the downhole testing tubing string is retrieved that the operators will know whether the data are sufficient and not corrupted. Similarly when operating some of the downhole testing tools from surface, such as tester valves, circulating valves, packers, samplers or perforating charges, the operators do not obtain a direct feedback from the downhole tools.

In this type of downhole testing operations, the operator can greatly benefit from having a two-way communication between surface and downhole. However, it can be difficult to provide such communication using a cable since inside the tubing string it limits the flow diameter and requires complex structures to pass the cable from the inside to the outside of the tubing. A cable inside the tubing is also an additional complexity in case of emergency disconnect for an offshore platform. Space outside the tubing is limited and a cable can easily be damaged. Therefore a wireless telemetry system is preferred.

A number of proposals have been made for wireless telemetry systems based on acoustic and/or electromagnetic communications. Examples of various aspects of such systems can be found in: U.S. Pat. No. 5,050,132; U.S. Pat. No. 5,056,067; U.S. Pat. No. 5,124,953; U.S. Pat. No. 5,128,901; U.S. Pat. No. 5,128,902; U.S. Pat. No. 5,148,408; U.S. Pat. No. 5,222,049; U.S. Pat. No. 5,274,606; U.S. Pat. No. 5,293,937; U.S. Pat. No. 5,477,505; U.S. Pat. No. 5,568,448; U.S. Pat. No. 5,675,325; U.S. Pat. No. 5,703,836; U.S. Pat. No. 5,815,035; U.S. Pat. No. 5,923,937; U.S. Pat. No. 5,941,307; U.S. Pat. No. 5,995,449; U.S. Pat. No. 6,137,747; U.S. Pat. No. 6,147,932; U.S. Pat. No. 6,188,647; U.S. Pat. No. 6,192,988; U.S. Pat. No. 6,272,916; U.S. Pat. No. 6,320,820; U.S. Pat. No. 6,321,838; U.S. Pat. No. 6,912,177; EP0550521; EP0636763; EP0773345; EP1076245; EP1193368; EP1320659; EP1882811; WO96/024751; WO92/06275; WO05/05724; WO02/27139; WO01/39412; WO00/77345; WO07/095111.

Tubing within a downhole environment can be constructed of a plurality of pipe sections that are connected together using threaded connections at both ends of the pipe sections. The pipe sections can have uniform or non-uniform pipe lengths. With respect to the non-uniform lengths, this is typically caused by the pipe sections being repaired by cutting part of the connection to re-machine the threads. The uniformity or non-uniformity of the pipe lengths can affect the way in which acoustic messages propagate along the tubing.

Because of the repetitive structure of the pipe sections, the characteristic of the acoustic propagation along pipe sections is such that the frequency response of the channel is complex. FIG. 3 shows an experimental and theoretical frequency response of a section of a tubing. The spectrum has numerous peaks and troughs. Given the spectrum and the use of a mono-carrier modulation scheme, choosing a peak for the carrier frequency of the transmitted modulated signal where noise is incoherent with the signal is advantageous in terms of signal to noise ratio. Choosing a carrier frequency around a locally flat channel response, i.e. no distortion, is advantageous to maximize the bit rate. In any case, choosing the carrier frequency in situ may be a requirement, and the process of choosing the right carrier frequency may take time and computing resources and should preferably be as simple as possible.

US 2006/0187755 by Robert Tingley discloses a method and system for communicating data through a drill string by transmitting multiple sets of data simultaneously at different frequencies. The Tingley reference attempts to optimize the opportunity of successful receipt despite the acoustic behavior of the drill string, thereby avoiding the problem of selecting a single frequency.

Moreover, U.S. Pat. No. 5,995,449 by Clark Robison et al. discloses a method and apparatus for communicating in a wellbore utilizing acoustic signals. However, the Robison et al. disclosure relates specifically to an apparatus and method for transmitting acoustic waves through the completion liquid as a transmission medium, rather than the tubing or pipe string.

In one method of “calibrating” the carrier frequency, a wireline probe is installed in a tubular string to communicate with acoustic modems (e.g., acoustic transceivers) interconnected in the tubular string. When initiated by an operator at the surface, the wireline probe prompts one of the acoustic modems to transmit a sweep of frequencies. See for example U.S. Patent Publication No. 2008/0180273

The wireline probe is positioned at another acoustic modem during the transmission of the frequency sweep, in order to detect characteristics of the received signals. The operator can then select which carrier frequency is optimum for transmission of messages between the two acoustic modems.

It will be appreciated that this method is time-consuming, requires installation and operation of the wireline probe and requires the services of a highly skilled operator. This method, and the method which requires prior knowledge of a particular carrier frequency, are also not suited for coping with changes in the well environment over time (which will also change the optimum carrier frequency), without repeating the expensive and complex operation of calibrating or changing the carrier frequency.

Techniques have been proposed for calibrating the carrier frequency without human intervention and without prior knowledge of the modulating carrier frequency. See for example U.S. Patent Publication No. 2008/0180273 which describes a method for detecting a usable downhole wireless telemetry system carrier frequency where a first telemetry device transmits one or more message(s) modulated on a carrier frequency; then checks whether a response to the message is received at the first telemetry device from a second telemetry device, and each time the response is not received, repeating the transmitting and checking steps with an incremented carrier frequency.

However, the robustness of the system is not based solely on the carrier frequency. For example, if the channel is chaotic within the band pass of the modulated signals, or depending upon the impulse response, the transmitted modulated signals are distorted by the channel. The distortion creates Inter Symbol Interference (“ISI”). When the received signals are decoded, the ISI causes bit errors. If the bit rate is increased, the ISI increases and the probability of having bit errors also increase. The channel may also attenuate the transmitted modulated signals. This attenuation decreases the Signal to Noise Ratio (i.e., “SNR”) which is defined as the power of the modulated signals over the noise power. The receiver decodes received modulated signals having a minimum SNR value. If the channel attenuates the modulated signals too much, the received signals cannot be properly decoded by the receiver. All these effects depend on the band pass of the modulated signals and affect the ability of the receiver to decode the encoded information. It should be pointed out that for mono-carrier modulations, the modulated signals are narrow band. Thus, a given carrier frequency can be usable, but not at a given bit rate.

In the downhole environment, it is difficult to increase the processing power of circuitry within the receiver due to the harshness of the downhole environment and limited power sources. For this reason, methodologies that use less processing complexity are desirable because such methodologies extend the use of the limited power sources.

There is a need for an acoustic modem that facilitates automatic synchronization of carrier frequency and/or bit rate of acoustic messages transmitted along at least a portion of a tubing section in a borehole for more reliable data transmission. There is also a need for enhancing the selection of an appropriate carrier frequency for the acoustic messages. It is to such an acoustic modem and method of transmission that the present disclosure is directed.

BRIEF SUMMARY OF THE DISCLOSURE

In one aspect, the present disclosure describes a method of transmitting data along tubing in a borehole. In the method, an acoustic message is transmitted by a first modem at a first location on the tubing at a first frequency and a first bit rate selected from a predetermined group of at least two frequencies and at least two bit rates. The acoustic message contains data. The acoustic message is received by a second modem at a second location on the tubing. The first frequency of the acoustic message is detected by receiver electronics of the second modem. The acoustic message is synchronized with the detected first frequency and in parallel with at least two bit rates by the receiver electronics of the second modem.

In one version, the frequency of the acoustic message is adjusted to one of the at least two frequencies in the predetermined group of at least two frequencies and at least two bit rates. The predetermined group of the at least two frequencies and the at least two bit rates comprises the first frequency and more than two further frequencies. The method can also further comprise the steps of iterating through the group of frequencies, and/or adjusting the bit rate of the acoustic message to a lower bit rate. The step of adjusting the bit rate can also follow adjustment of the frequency of the acoustic message.

In other versions, the method also includes a step of re-transmitting the data within the acoustic message received by the second modem to a third modem, and/or determining whether the step of synchronization is successful utilizing a predetermined selection algorithm selected from a group of at least one signal quality parameter consisting of signal distortion, signal strength, ambient noise, signal-to-interference noise ratio, signal-to-noise ratio, channel response time, signal amplitude, and signal auto-correlation.

In another version, the present disclosure describes a method in which an acoustic message is converted into an electrical signal by a transceiver assembly of an acoustic modem attached to a tubing within a borehole. The acoustic message includes a synchronization frame and is transmitted at a first frequency and a first bit rate selected from a group of multiple frequencies and bit rates. The electrical signal is received by receiver electronics of the acoustic modem and the receiver electronics synchronizes, in parallel, a synchronization frame of the electrical signal with at least two frequencies and at least two bit rates of the group of multiple frequencies and bit rates. The multiple frequencies and bit rates within the group can be predetermined.

In another version, the present disclosure describes an acoustic modem for communication in a network of acoustic modems via a communication channel. The acoustic modem is provided with a transceiver assembly adapted to convert acoustic messages into electrical signals. The acoustic modem is also provided with transceiver electronics and a power supply supplying power to the transceiver assembly and the transceiver electronics. The transceiver electronics is provided with transmitter electronics and receiver electronics. The transmitter electronics cause the transceiver assembly to send acoustic signals into the communication channel while the receiver electronics comprising at least one microcontroller. The least one microcontroller is adapted to execute instructions to (1) enable the receiver electronics to receive electrical signals indicative of the acoustic message from at least one other acoustic modem via the transceiver assembly, (2) estimate a carrier frequency of the electrical signals by analyzing an estimation frame of the electrical signals, (3) estimate a starting time of a data frame of the electrical signals by synchronizing with a synchronization frame of the acoustic message in parallel with at least two bit rates, and (4) decode the data frame.

In yet other aspects, the at least one microcontroller of the receiver electronics enables the transmitter electronics to transmit an acknowledgement, and the at least one microcontroller executes instructions to cause (2) and (3) to execute sequentially.

In yet another version, the present disclosure describes an acoustic modem for communication in a network of acoustic modems via a communication channel. The acoustic modem is provided with a transceiver assembly, transceiver electronics, and a power supply. The transceiver assembly is adapted to convert electrical signals into acoustic messages. The transceiver electronics is provided with transmitter electronics and receiver electronics. The transmitter electronics and the receiver electronics are coupled to the transceiver assembly. At least one microcontroller is also provided to execute instructions to (1) enable the transmitter electronics to enter a training phase where the transmitter electronics transmit a training message having an estimation frame, a synchronization frame and a data frame to the transceiver assembly, and (2) enable the transmitter electronics to enter a data communication phase where the transmitter electronics transmits a data message to the transceiver assembly having a synchronization frame and a data frame without an estimation frame; and

In yet another version, the acoustic modem may include receiver electronics having at least one microcontroller executing instructions to (1) receive the electrical signal indicative of the acoustic message; (2) synchronize, in parallel, a synchronization frame of the electrical signal with at least two frequencies and at least two bit rates of a group of multiple frequencies and bit rates; and (3) decode the data frame.

In yet another version, the acoustic modem may include, transmitter electronics coupled to the transceiver assembly and including at least one microcontroller adapted to execute instructions to generate a sequence of electrical signals directed to the transceiver assembly with different carrier frequencies and with the sequence ordered based upon a model of a particular, planned tubing for a borehole.

In another aspect, the present disclosure describes a computer system having one or more input device, one or more output device, and one or more non-transitory computer readable medium storing instructions for (1) receiving a priori information of a particular, planned tubing for a borehole from the one or more input device, (2) predicting optimal carrier frequencies for acoustic communication utilizing the particular, planned tubing as a communication channel between two or more modems, and (3) outputting information to the output device indicative of the optimal carrier frequencies. The computer system is also provided with at least one processor in communication with the one or more non-transitory computer readable medium for executing the instructions based upon a message from the one or more input device, and one or more power supply supplying power to the input device, the output device, the one or more non-transitory computer readable medium and the one or more processor.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:

FIG. 1 shows a schematic view of an acoustic telemetry system according to an embodiment of the present invention;

FIG. 2 shows a schematic of an acoustic modem as used in accordance with the embodiment of FIG. 1;

FIG. 3 depicts an acoustic frequency response of a tubing section;

FIG. 4A illustrates a flow diagram of a method according to an embodiment of the present invention;

FIG. 4B illustrates a flow diagram of another method according to an embodiment of the present invention;

FIGS. 5A and 5B show flow diagrams of a receiver architecture for use in an embodiment of the present invention in which receiver electronics of the acoustic modem attempts to synchronize on combinations of different bit rates and carrier frequencies in parallel;

FIG. 6A shows a training message generated by version(s) of the presently disclosed and claimed inventive concepts;

FIG. 6B shows a data message generated by version(s) of the presently disclosed and claimed inventive concepts;

FIG. 7 is a flow diagram of a receiver architecture for use in an embodiment of the presently disclosed and claimed inventive concepts;

FIG. 8 is a block diagram of a computer system that can be used to execute a pipe communication algorithm in accordance with the present disclosure;

FIG. 9 is a side elevation view of a section of tubing that can be modeled by the pipe communication algorithm in accordance with the present disclosure;

FIG. 10 is an exemplary simulated transfer function based upon a priori information for a particular, planned tubing to be modeled formed of pipe sections having a pipe length of 9.5 m, a connection length of 0.35 m, a connection outer cross-section of 4.5 inches, inner diameter 2.75 inches, and a pipe outer diameter of 3.5 inches; and

FIG. 11 is another simulated transfer function of two exemplary tubing sections showing a difference in attenuation between one tubing formed of pipe sections having a uniform pipe length, and another tubing having pipe lengths of differing sizes within a standard deviation of 0.15 m.

FIG. 12 is another exemplary simulated transfer function based upon a priori for a particular, planned tubing illustrating locations of band stops within the frequency domain.

DETAILED DESCRIPTION

The present invention is particularly applicable to testing installations such as are used in oil and gas wells or the like. FIG. 1 shows a schematic view of such a system. Once a well 10 has been drilled through a formation, the drill string can be used to perform tests, and determine various properties of the formation through which the well has been drilled. In the example of FIG. 1, the well 10 has been lined with a steel casing 12 (cased hole) in the conventional manner, although similar systems can be used in unlined (open hole) environments. In order to test the formations, it is preferable to place testing apparatus in the well close to the regions to be tested, to be able to isolate sections or intervals of the well, and to convey fluids from the regions of interest to the surface. This is commonly done using a jointed tubular drill pipe, drill string, production tubing, sections thereof, or the like (collectively, tubing 14) which extends from well-head equipment 16 at the surface down inside the well 10 to a zone of interest. The well-head equipment 16 can include blow-out preventers and connections for fluid, power and data communication.

A packer 18 is positioned on the tubing 14 and can be actuated to seal the borehole around the tubing 14 at the region of interest. Various pieces of downhole test equipment (collectively, downhole equipment 20) are connected to the tubing 14 above or below the packer 18. Such downhole equipment 20 may include, but is not limited to: additional packers; tester valves; circulation valves; downhole chokes; firing heads; TCP (tubing conveyed perforator) gun drop subs; samplers; pressure gauges; downhole flow meters; downhole fluid analyzers; and the like.

In the embodiment of FIG. 1, a sampler 22 is located above the packer 18 and a tester valve 24 is located above the packer 18. The downhole equipment 20 is connected to an acoustic modem 25Mi+1 which can be mounted in a gauge carrier 28 positioned between the sampler 22 and the tester valve 24. The acoustic modem 25Mi+1, operates to allow electrical signals from the downhole equipment 20 to be converted into acoustic signals for transmission to the surface via the tubing 14, and to convert acoustic tool control signals from the surface into electrical signals for operating the downhole equipment 20. The term “data,” as used herein, is meant to encompass control signals, tool status, and any variation thereof whether transmitted via digital or analog signals.

FIG. 2 shows a schematic of the acoustic modem 25Mi+1 in more detail. The acoustic modem 25Mi+1 comprises a housing 30 supporting a transceiver assembly 32 which can be a piezo electric actuator or stack, and/or a magnetorestrictive element which can be driven to create an acoustic signal in the tubing 14 when the acoustic modem 25Mi+1 is mounted in the gauge carrier 28. The acoustic modem 25Mi+1 can also include an accelerometer 34 and/or an additional transceiver assembly 35 for receiving acoustic signals. Where the acoustic modem 25Mi+1 is only required to receive acoustic messages, the transceiver assembly 32 may be omitted. The acoustic modem 25Mi+1 also includes transmitter electronics 36 and receiver electronics 38 located in the housing 30 and power is provided by means of a battery, such as a lithium battery 40. Other types of power supply may also be used.

The transmitter electronics 36 are arranged to initially receive an electrical output signal from a sensor 42, for example from the downhole equipment 20 provided from an electrical or electro/mechanical interface. Such signals are typically digital signals which can be provided to a micro-controller 43 which modulates the signal in one of a number of known ways PSK, QPSK, QAM, and the like. The micro-controller 43 can be implemented as a single micro-controller or two or more micro-controllers working together. In any event, the resulting modulated signal is amplified by either a linear, or non-linear, amplifier 44 and transmitted to the transceiver assembly 32 so as to generate an acoustic signal (which is also referred to herein as an acoustic message) in the material of the tubing 14.

The acoustic signal passes along the tubing 14 as a longitudinal and/or flexural wave comprises a carrier signal with an applied modulation of the data received from the sensors 42. The acoustic signal typically has, but is not limited to, a frequency in the range 1-10 kHz, preferably in the range 2-5 kHz, and is configured to pass data at a rate of, but is not limited to, about 1 bps to about 200 bps, preferably from about 5 to about 100 bps, and more preferably about 50 bps. The data rate is dependent upon conditions such as the noise level, carrier frequency, Inter Symbol Interference and the distance between the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1. A preferred embodiment of the present disclosure is directed to a combination of a short hop acoustic modems 25Mi−1, 25M and 25Mi+1 for transmitting data between the surface and the downhole equipment 20, which may be located above and/or below the packer 18. The acoustic modems 25Mi−1 and 25M can be configured as repeaters of the acoustic signals. The system may be designed to transmit data as high as 200 bps. Other advantages of the present system exist.

The receiver electronics 38 of the acoustic modem 25Mi+1 are arranged to receive the acoustic signal passing along the tubing 14 produced by the transmitter electronics 36 of the acoustic modem 25M. The receiver electronics 38 are capable of converting the acoustic signal into an electric signal. In a preferred embodiment, the acoustic signal passing along the tubing 14 excites the transceiver assembly 32 so as to generate an electric output signal (voltage); however, it is contemplated that the acoustic signal may excite the accelerometer 34 or the additional transceiver assembly 35 so as to generate an electric output signal (voltage). This signal is essentially an analog signal carrying digital information. The analog signal is applied to a signal conditioner 48, which operates to filter/condition the analog signal to be digitalized by an A/D (analog-to-digital) converter 50. The A/D converter 50 provides a digitalized signal which can be applied to a microcontroller 52. The microcontroller 52 is preferably adapted to demodulate the digital signal in order to recover the data provided by the sensor 42, or provided by the surface. The type of signal processing depends on the applied modulation (i.e. PSK, QPSK, QAM, and the like).

The acoustic modem 25Mi+1 can therefore operate to transmit acoustic data signals from sensors 42 in the downhole equipment 20 along the tubing 14. In this case, the electrical signals from the downhole equipment 20 are applied to the transmitter electronics 36 (described above) which operate to generate the acoustic signal. The acoustic modem 25Mi+1 can also operate to receive acoustic control signals to be applied to the downhole equipment 20. In this case, the acoustic signals are demodulated by the receiver electronics 38 (described above), which operate to generate the electric control signal that can be applied to the downhole equipment 20.

Returning to FIG. 1, in order to support acoustic signal transmission along the tubing 14 between the downhole location and the surface, a series of the acoustic modems 25Mi−1 and 25M, etc. may be positioned along the tubing 14. The acoustic modem 25M, for example, operates to receive an acoustic signal generated in the tubing 14 by the acoustic modem 25Mi−1 and to amplify and retransmit the signal for further propagation along the tubing 14. The number and spacing of the acoustic modems 25Mi−1 and 25M will depend on the particular installation selected, for example on the distance that the signal must travel. A typical spacing between the acoustic modems 25Mi−1, 25M, and 25Mi+1 is around 1,000 ft, but may be much more or much less in order to accommodate all possible testing tool configurations. When acting as a repeater, the acoustic signal is received and processed by the receiver electronics 38 and the output signal is provided to the microcontroller 52 of the transmitter electronics 36 and used to drive the transceiver assembly 32 in the manner described above. Thus an acoustic signal can be passed between the surface and the downhole location in a series of short hops.

The role of a repeater is to detect an incoming signal, to decode it, to interpret it and to subsequently rebroadcast it if required. In some implementations, the repeater does not decode the signal but merely amplifies the signal (and the noise). In this case the repeater is acting as a simple signal booster. However, this is not the preferred implementation selected for wireless telemetry systems of the present invention.

The acoustic modems 25M, 25Mi−1, 25Mi−2, and 25Mi+2 will either listen continuously for any incoming signal or may listen from time to time.

The acoustic wireless signals, conveying commands or messages, propagate in the transmission medium (the tubing 14) in an omni-directional fashion, that is to say up and down. It is not necessary for the acoustic modem 25Mi+1 to know whether the acoustic signal is coming from the acoustic modem 25M above or an acoustic modem 25Mi+2 (not shown) below. The direction of the acoustic message is preferably embedded in the acoustic message itself. Each acoustic message contains several network addresses: the address of the acoustic modem 25Mi−1, 25M or 25Mi+1 originating the acoustic message and the address of the acoustic modem 25Mi−1, 25M or 25Mi+1 that is the destination. Based on the addresses embedded in the acoustic messages, the acoustic modem 25Mi−1 or 25M functioning as a repeater will interpret the acoustic message and construct a new message with updated information regarding the acoustic modem 25Mi−1, 25M or 25Mi+1 that originated the acoustic message and the destination addresses. Acoustic messages will be transmitted from acoustic modem 25Mi−1 to 25M and may be slightly modified to include new network addresses.

Referring again to FIG. 1, the acoustic modem 25Mi−2 is provided at surface, such as at or near the well-head equipment 16 which provides a connection between the tubing 14 and a data cable or wireless connection 62 to a control system 64 that can receive data from the downhole equipment 20 and provide control signals for its operation.

In the embodiment of FIG. 1, the acoustic telemetry system is used to provide communication between the surface and a section of the tubing 14 located downhole.

Full-Parallel Synchronization

A preferred embodiment of the present disclosure is based on a protocol in which the transmitter electronics 36 of one of the acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 transmits a message (i.e., a control signal or data signal) belonging to a predetermined set S_(f) of N frequencies (F₁, F₂, F₃, F₄, . . . F_(n)), and the receiver electronics 38 of another one of the acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 synchronizes in parallel on the predetermined set S_(f) of N frequencies until the communication succeeds. The receiver electronics 38 of the acoustic modem 25Mi+1, for example, simultaneously tries to demodulate the incoming signals transmitted by the acoustic modem 25M on the predetermined frequencies S_(f). The protocol is illustrated in FIG. 4A, in which S_(f) is shown to comprise four frequencies F₁-F₄, however, the predetermined set of frequencies may include much more or less. A scheme of the parallel receiver is shown in FIGS. 5A and 5B and provides the advantage of not requiring circuitry for frequency detection.

In the example illustrated in FIG. 4A, the transmitter electronics 36 of the acoustic modem 25M, for example, initially transmits a signal at frequency F₁. The receiver electronics 38 of the acoustic modem 25Mi+1 attempts to synchronize at multiple frequencies, F₁-F₄, but due to attenuation or distortion of the signal at this frequency, is unable to synchronize with this signal on F₁ and so does not send any acknowledgement signal back to the transmitter electronics 36 of the acoustic modem 25M. When starting to transmit at a given frequency, the transmitter electronics 36 of the acoustic modem 25M starts a timing routine. If no acknowledgement is received from the receiver electronics 38 of the acoustic modem 25Mi+1 within a predetermined time interval, the transmitter electronics 36 of the acoustic modem 25M times out and switches to the next frequency F₂. This process is preferably repeated until an acknowledgement signal is received from the receiver electronics 38 of the acoustic modem 25Mi+1 on the same frequency, at which time the transmitter electronics 36 of the acoustic modem 25M begins data transmission. One advantage of the parallel synchronization illustrated in the example of FIG. 4A is the robustness of the process, and the removal of the need for frequency detection. In the example of FIG. 4A, synchronization occurs at frequency F₃. It is contemplated that while one carrier frequency may be chosen for transmission from one of the acoustic modems 25M to another one of the acoustic modems 25Mi+1, a same or different second carrier frequency may be chosen for transmission from the acoustic modem 25Mi−1 to the acoustic modem 25M, for example.

The selection of an initial transmission frequency is preferably chosen from a set of frequencies based on past experience, but may also include an automatic mechanism at the beginning of the communication. This mechanism could consist in having all the transmitter electronics 36 of the acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 transmitting frequency sweeps at a predetermined time and all the receiver electronics 38 in the tubing 14 recording the incoming frequency sweeps, then determining the N best frequencies based on quality indicators such as amplitude, signal-to-noise ratio and spectrum flatness. The determination of the initial and subsequent transmission frequencies can also be determined based upon a priori knowledge of the tubing 14, as will be discussed in more detail below.

Based on the spectral estimate of the communication channel in various cases and assuming the set S_(f) is well chosen, it is likely that there is at least one carrier frequency out of N (N being small, such as 4 or 5, but may be much more) with limited attenuation and distortion.

FIGS. 5A and 5B shows schematically a receiver architecture of the receiver electronics 38 used for parallel synchronization. The receiver architecture corresponds to the signal processing, preferably implemented in the microcontroller 52 of the receiver electronics 38 depicted in FIG. 2. After the analog signal is digitalized by the A/D converter 50, the resulting digitalized signal can be simultaneously demodulated by the microcontroller 52 on the predetermined set of frequencies belonging to S_(f). The demodulation process preferably comprises a synchronization process 140 and a decoding process 142.

In the synchronization process 140 as depicted in FIGS. 5A and 5B, the microcontroller 52 simultaneously attempts to synchronize on the predetermined set of frequencies S_(f) and a predetermined set S_(b) of K bit rates (B₁, B₂, . . . ,B_(k)). Where the incoming signal only has one frequency, the microcontroller 52 attempts to synchronize on multiple frequencies, but may only succeed to synchronize on this signal frequency. The synchronization process can be based on correlation, where parallel synchronization consists of multiple, simultaneous correlations. If the synchronization is successful on the synchronized frequency, the beginning of the received signal is well known as well as its frequency. In the decoding process 142, the modulated signal is decoded and the data recovered. Where the incoming signal is transmitted on multiple frequencies, the microcontroller 52 selects the best frequency based on the highest correlation ratio and proceeds to decode the data on the best frequency.

In the example of FIG. 4A, the acoustic messages are all transmitted at the same bit rate and the receiver electronics 38 tries to synchronize on different frequencies at a single given bit rate. In another embodiment of the present disclosure as depicted in FIG. 4B, the bit rate can be varied. If the signal channel is unusually very noisy and none of the transmitted signals is recovered by the receiver electronics 38, the system of FIG. 4A will not work. In order to avoid this, the receiver electronics 38 can also synchronize at a lower bit rate for each of the frequencies belonging to S_(f). In other words, the receiver electronics 38 can be configured to simultaneously synchronize on multiple different carrier frequencies S_(f) as well as on a predetermined set of K bit rates (B₁, B₂, . . . ,B_(k)). Different combinations of carrier frequencies and bit rates are attempted by the transmitter electronics 36 until an acknowledgement is received during a time-out period. Once an acknowledgement is received, a preferred combination of carrier frequency and bit rate is selected.

As shown in FIG. 4B, the transmitter electronics 36 will preferably first try to transmit acoustic messages at a high bit rate. In case of failure, the transmitter electronics 36 will transmit the acoustic messages at successively lower bit rates, which is shown in FIG. 4B as a “low bit rate”. Since the energy per bit becomes higher as the bit rate decreases, the bit energy-to-noise ratio (Eb/N₀) is increased. In addition, since the signal bandwidth is reduced, then there is less ISI and the received acoustic signal is less distorted by the channel. Though this adds more complexity to the receiver electronics 38 and decreases the data rate, the communication becomes more robust.

In the example depicted in FIG. 4B, the number of available carrier frequencies and available bit rates can vary so long as the available carrier frequencies and available bit rates are greater than one. This is represented in certain parts of this document as N_(f) carrier frequencies and N_(br) bit rates where N_(f) and N_(br) are greater than 1.

The time out period can vary and depends on the duration in which the acknowledgement is expected to be received if the acoustic message was properly decoded. Various factors can be used to determine the time out period, such as the time duration of the acoustic message and/or the distance between the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1. The time out period can be the length of time that it takes for the receiver electronics 38 to transmit the acknowledgement to the transmitter electronics 36. For example, if the receiver electronics 38 is transmitting bits at 50 bits/second, and the acknowledgement includes 150 bits, then the time out period can be approximately 3 seconds. Further, the acknowledgement should be as short as possible to optimize the performance of the system.

In general, the communication channel between a pair of the acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 is assumed to be symmetric. As such, the acoustic signal used for the acknowledgement(s) may be transmitted using the same carrier frequency and/or bit rate as that of the training message(s). In this instance, after a carrier frequency and/or bit rate have been selected, the pair of acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 may store the values for the selected bit rate and/or carrier frequency for communicating with each other. Thus, for future transmissions, in this embodiment data will be transmitted between a particular pair of the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 using the selected bit rate and carrier frequency.

In the example shown in FIG. 4A, the transmitter electronics 36 does not receive an acknowledgement within the time out period for carrier frequencies F₁ and F₂ which means the receiver electronics 38 either did not demodulate the acoustic signal, or the acoustic signal was demodulated but the signal quality was insufficient. The sufficiency of the signal quality can be determined by setting a minimum threshold for the signal quality and comparing the minimum threshold to a quantitative evaluation of the quality, such as a bit error rate.

In the example shown in FIG. 4B, once all of the available carrier frequencies have been tried for a particular bit rate, another bit rate is selected and then the transmitter electronics 36 of the acoustic modem 25M, for example, cycles through the available carrier frequencies.

Higher bit rates are typically preferred over lower bit rates. If all the available carrier frequencies have been tested and no demodulation has been successful at a high bit rate or the quality of the received signals is insufficient, the transmitter electronics 36 successively transmits low bit-rate signals at different carrier frequencies until the receiver electronics 38 demodulates the acoustic signal at a sufficient quality and provides the acknowledgement.

For mono-carrier modulations, the quality of the received modulated signals, which is often measured by the Signal to Interference and Noise Ratio (SINR), is directly related to the characteristics of the channel within the band pass of the modulated signal. The SINR is defined as the signal power over the interference and noise power.

If the quality measure of a received signal is higher than a quality threshold, the signal quality is sufficient, e.g., the signal being decoded with a bit error rate BER which is below a BER target. The BER target can be specified by the user of the communication system. It is for example equal to 10⁻³ or 10⁻⁴. The quality threshold can be derived from the BER target.

The methods set forth in FIGS. 4A and 4B are only examples. One embodiment of the present disclosure corresponds to the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 being able to communicate at several frequencies and several bit rates and the frequencies and the bit rates can be tested in any order. The quality criteria can be derived from the SINR or any other measure, such as the SNR, which indicate the communication performance of the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1. Several quality criteria can even be taken into consideration.

In the example discussed above, it is assumed that the communication channel between the transmitter electronics 36 and the receiver electronics 38 is symmetric. However, it should be understood that the training phase can be adapted to non-symmetric channels. In the case of a non-symmetric channel, a carrier frequency and/or a bit rate that works from the transmitter electronics 36 to the receiver electronics 38 does not necessarily work from the receiver electronics 38 to the transmitter electronics 36. Therefore, if the receiver electronics 38 successfully demodulates an acoustic message transmitted by the transmitter electronics 36, the acknowledgment transmitted from the receiver electronics 38 to the transmitter electronics 36 will not necessarily be successfully demodulated by the transmitter electronics 36 or the quality of the signal might be too poor.

In reality, the communication channel is partially symmetric. Locally there might be some slight differences due to the electronics, and/or the transceiver assembly 32, the transmitter electronics 36 and the receiver electronics 38 are not matched. Therefore, the transfer functions from the transmitter electronics 36 to the receiver electronics 38 and vice-versa are slightly different and the noise seen by both might also be different.

Since however the differences are normally small, in general, a carrier frequency and bit rate that works when transmitting from the transmitter electronics 36 to the receiver electronics 38 also works when transmitting from the receiver electronics 38 to the transmitter electronics 36. Thus, in one embodiment, a carrier frequency and bit rate that works in both directions is selected. However, different carrier frequencies and/or bit rates for communication in between the transmitter electronics 36 and the receiver electronics 38 can be selected.

The modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 have to be able to communicate at at least one carrier frequency and one bit rate in accordance with the presently disclosed inventive concepts set forth in FIG. 4B. The transmitter electronics 36 can generate mono-carrier modulated signals at different frequencies and/or different bit rates by preferably using digital and/or analog circuitry to generate a desired waveform to be provided to the transceiver assembly 32. However, interpreting and/or decoding the acoustic signal by the receiver electronics 38 can be far more complicated since the carrier frequency and the bit rate may be unknown.

An exemplary receiver architecture for implementing the inventive concepts set forth in FIG. 4B is shown in FIGS. 5A and 5B. In the embodiment of FIG. 5B the receiver electronics 38 synchronizes and decodes an acoustic signal having an unknown carrier frequency and bit rate with the at least one microcontroller 52 synchronizing with the acoustic signal utilizing multiple combinations of carrier frequency and bit rate in parallel. The receiver architecture of FIG. 5B has a full parallel synchronization state 150 and a decoding state 152. The full parallel synchronization state 150 estimates the carrier frequency and the bit rate of the acoustic signal, as well as accurately estimates the start time of a data frame contained in the acoustic signal by synchronizing at N_(f) frequencies and N_(br) bit rates in parallel. The microcontroller 52 stores a plurality of predetermined synchronization frames that are calculated using different combinations of N_(f) and N_(br). During the full parallel synchronization state 150, combinations of N_(f)*N_(br) correlation coefficients are calculated in real time. The normalized correlation coefficients typically have a range between 0 and 1.

Assuming that an acoustic signal is received, the receiver electronics 38 synchronizes at the frequency and bit rate which give the maximum value for the correlation coefficient to estimate the carrier frequency and bit rate of the acoustic signal. In particular, the receiver electronics 38 is directed to monitor the correlation coefficients and wait for a maximum of the correlation coefficients. The time instant of this maximum, called synchronization time, corresponds to the time of the last symbol of the synchronization frame. The output of the full parallel synchronization state 150 is preferably (1) a start time of a data frame of the acoustic signal, (2) an estimated frequency of the acoustic signal, and (3) an estimated bit rate of the acoustic signal.

The start time of the data frame of the acoustic signal, the estimated frequency of the acoustic signal, and the estimated bit rate of the acoustic signal are provided to the decoding state 152, which decodes the data frame of the acoustic signal.

Reduced-Parallel Synchronization

Shown in FIG. 7 is another receiver architecture which includes a training phase 200 and a data communication phase 202, which can be implemented as separate threads, for example. During the training phase 200, the receiver electronics 38 is adapted to receive and decode a training message 204 having an unknown carrier frequency and bit rate, which is shown by way of example in FIG. 6A. During the data communication phase 202, the receiver electronics 38 is adapted to receive and decode a data message 208 having a known carrier frequency but unknown bit rate, which is shown by way of example in FIG. 6B.

The training message 204 utilized in embodiments of the present disclosure has been modulated using an available carrier frequency and bit rate and encoded with data using a suitable encoding scheme as discussed above. In general, the training message 204 is generally composed of at least three parts. The first part is called a synchronization frame 210 and is composed of a number of symbols N_(s). The second part is called a data frame 212 and is composed of a number of data symbols N_(d).

The training message 204 also includes an estimation frame 214 to permit the receiver electronics to estimate the carrier frequency by analyzing the estimation frame 214 of the training message 204. The estimation frame 214 is prior to the synchronization frame 210 and preferably includes a sinusoid at the same frequency as the carrier frequency of the remaining part of the modulated signal. The length of the estimation frame 214 ranges typically from a few hundreds of milliseconds to a few seconds depending upon the targeted performance of the modulation stage. The estimation frame 214 having a sinusoid at the same frequency as the carrier frequency may be referred to herein as a “sine prefix”.

As shown in FIG. 6B, the data message 208 includes the synchronization frame 210 and the data frame 212, but does not include the estimation frame 214 since the carrier frequency of the data message 208 is known. As discussed herein, the estimation frame 214 can be omitted once the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 have entered the data communication phase 202.

FIG. 7 is a logic flow diagram of the receiver electronics 38 constructed in accordance with the presently disclosed and claimed embodiments. In this version, the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 include the training phase 200 and the data communication phase 202. The training phase 200 is typically executed prior to the data communication phase 202 for initializing the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 to communicate with each other. During the training phase 200, the training message 204 includes the estimation frame 214 (as shown in FIG. 6A) to permit the microcontroller 52 to estimate the carrier frequency by analyzing the estimation frame 214 of the training message 146. The estimation frame 214 is prior to the synchronization frame 210 and preferably defines the sine prefix, although signals other than a sinusoid can be used. During the training phase 200 the length of the estimation frame 214 can be increased to also increase the robustness of the process.

In this version, the receiver electronics 38 is programmed to include at least three states in the training phase 200, i.e., a frequency estimation state 220, a reduced parallel synchronization state 222 and a decoding state 224. The frequency estimation state 220 analyzes the estimation frame 214 to detect whether the frequency of the estimation frame 214 includes a carrier frequency within a predetermined set of N_(f) carrier frequencies. The frequency estimation state 220 can accomplish this in any suitable manner, such as by using signal processing tools such as fast Fourier transforms, moving averages and/or filterings and then comparing the output of the signal processing tools to the predetermined set of carrier frequencies N_(f).

When an acoustic signal of a predetermined set of N_(f) frequencies has been detected; the frequency estimation state 220 branches to the reduced parallel synchronization state 222 at the detected carrier frequency. The reduced parallel synchronization state 222 synchronizes in parallel at the detected carrier frequency and at at least two bit rates. The output of the reduced parallel synchronization state 222 is the signal bit rate in addition to the start time of the data frame. The reduced parallel synchronization state 222 can be implemented by calculating the correlation coefficients and looking for a maximum as discussed above.

After synchronization, the receiver electronics 38 branches into the decoding state 224 and decodes the data frame 212 using the detected frequency and the detected bit rate. The decoding state 224 is similar to the decoding states discussed above. If the decoding state 224 successfully decodes the data frame 212 to a particular quality level, then an acknowledgement is transmitted to the transmitter electronics 36 and the carrier frequency and the bit rate is stored for future communication during the data communication phase 202. If the decoding state is unsuccessful, then an acknowledgement is preferably not sent and the transmitter electronics 36 sends out another acoustic signal having a different combination of carrier frequency and bit rate as discussed above. This process then repeats for each set of acoustic modems 25Mi−2, 25Mi−1, 25M or 25Mi+1 to be initialized on the tubing.

Once the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 have been initialized, the data communication phase 202 is actuated as shown in FIG. 7. The data communication phase 202 is preferably implemented as the reduced parallel synchronization state 222 and the decoding state 224. In the data communication phase 202, the transmitter electronics 36 of the transmitter is adapted to output data messages 208 as shown in FIG. 6B. The data messages 208 include the synchronization frame 210 and the data frame 212 as discussed above. In particular, the data message 208 preferably does not include the estimation frame 214 of the training message 204 since the carrier frequencies are already known. In one embodiment, the reduced parallel synchronization state 222 calculates multiple correlation coefficients using a same carrier frequency and multiple bit rates simultaneously to determine the bit rate of the acoustic signal although other synchronization methodologies could be used. When an acoustic signal is received at this frequency, and if the synchronization is successful, the data communication phase 202 goes into the decoding state 224 at the same carrier frequency and at the detected bit rate.

The training phase 200 and the data communication phase 202 are used for different purposes. In particular, assuming that the transmitter electronics 36 of one of the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1 is transmitting data to the receiver electronics 38 of another one of the acoustic modems 25Mi−2, 25Mi−1, 25M and 25Mi+1. The transmitter electronics 36 first selects the communication frequency and bit rate to communicate between the transmitter electronics 36 and the receiver electronics 38. Then, the transmitter electronics 36 initiates the training phase 200 such as the one described above. The receiver electronics 38 uses the training phase 200 to select a proper frequency and bit rate by estimating the frequency and bit rate of the incoming training messages 204, demodulating the training messages 204 and sending an acknowledgement message upon successful demodulation. After a proper frequency and bit rate have been selected, the transmitter electronics 36 and/or the receiver electronics 38 initiates the data communication phase 202. Subsequently, the transmitter electronics 36 then transmits data to the receiver electronics through data messages 208 at the selected frequency and/or bit rate. Since the carrier frequency is set, the receiver electronics 38 uses the data communication phase 202 to demodulate incoming data signals at the selected frequency.

Assuming the channel is time-constant, the training phase 200 is preferably only performed once at the beginning of the drill stem test DST job, for example. Though it cannot be too long, its time duration may not be critical for the overall performance of the system. On the contrary, the training phase 200 is preferably robust. The robustness of the frequency estimation state 220 can be increased by lengthening the size of the estimation frame 214. However, in the data communication phase 202, the estimation frame 214 is preferably omitted in the data messages 208 to enhance the data communication rate between the transmitter electronics 36 and the receiver electronics 38 and also because there may not be any requirement for frequency detection. Thus, in a preferred embodiment, during the training phase 200, the transmitter electronics 36 generates the training message 204 with the estimation frame 214, and during the data communication phase 202, the transmitter electronics generates the data message 208 without the estimation frame 214.

The complexity of the receiver electronics 38 is the complexity of the most complex state and the complexity determines how much processing power and battery power is needed to optimize the bit rate by using the highest possible bit rate. As will be discussed below, the reduced parallel synchronization state 222 is much less complex than the full parallel synchronization state 150. In particular, during the reduced parallel synchronization state 222, N_(br) correlation coefficients are calculated simultaneously. The complexity of this state is therefore N*N_(br) where N stands for the assumed complexity of the synchronization state. Since this is the most complex state, the complexity of the transmitter electronics 36 and the receiver electronics 38 is N*N_(br) where N stands for the assumed complexity of the synchronization state at one frequency and bit rate. Assuming there are 10 possible carrier frequencies, and two possible bit rates, the reduced parallel synchronization state 222 has a complexity of 2N. The full-parallel synchronization state 150, on the other hand simultaneously attempts to synchronize in parallel on all possible bit rates and all possible carrier frequencies resulting in a complexity of N*N_(br).=20N. Thus, the receiver electronics 38 utilizing the reduced parallel synchronization state 222 is less complex by a factor of 10 thereby utilizing less processing power and battery power once the receiver electronics 38 has entered the data communication phase 202.

Prediction of Optimal Communication Frequencies

In another aspect, the efficiency of the transmitter electronics 36 in selecting an appropriate carrier frequency can be improved by using a pipe communication algorithm running on a computer system 288 shown by way of example in FIG. 8. The pipe communication algorithm receives and analyzes a priori information for a particular, planned tubing 14 in order to predict the optimal communication frequencies to be used by the acoustic modems 25Mi+2, 25Mi+1, 25M and 25Mi−1 once installed on the tubing 14 and during the DST operations. In general, a priori information, as used herein, is prior knowledge about the tubing 14, such as an average pipe section length, a statistical distribution of pipe length, or planned DST operations which may affect an acoustic channel formed by the tubing 14 in a predictable way. Using the a priori information, the presently disclosed concepts produce an acoustic transfer function 290 (an example of which is shown in FIG. 10) of the tubing 14 to determine communication frequencies and expected attenuation within band pass zones of the acoustic transfer function 290, for example. The identification of communication frequencies within the band pass zones, as well as the expected attenuation can be used in selecting optimal carrier frequencies and their order. The transmitter electronics 36 can then be programmed with, and/or utilize the optimal carrier frequencies in selecting the order of carrier frequencies within the training phases discussed above for full parallel or reduced parallel synchronization, thereby reducing the time and energy needed for selecting an appropriate carrier frequency.

The computer system 288 is provided with one or more processor 292, one or more non-transitory computer readable medium 294, one or more output device 296, one or more input device 297, and one or more communication device 298, and one or more power supply 299.

The one or more non-transitory computer readable medium 294 can be implemented in a variety of ways, such as a random access memory, a read-only memory, an EEPROM, an optical disk, a hard drive, or the like. In general, the non-transitory computer readable medium 294 stores the pipe communication algorithm and also stores the a priori information with respect to the tubing 14, such that the pipe communication algorithm can be executed by the processor 292 to produce and/or store the acoustic transfer function 290 as discussed above.

The one or more processor 292 will be referred to hereinafter as “the processor 292” for purposes of brevity, however it should be understood that the processor 292 can be implemented as a single processor or multiple processors working independently or together to perform the functions discussed herein. The processor 292 can be implemented as one or more microcontroller, central processing unit, digital signal processor, field programmable gate array, or the like.

The input device 297 can be used by a person or operator to input data and/or control the functioning of the processor 292. Exemplary input devices 297 include a keyboard, mouse, trackball, touchscreen, USB port, optical drive, or the like. The output device 296 serves to convey information to the person or operator (as well as others) regarding the operations of the processor 292. Exemplary output devices 296 include a monitor, a printer, USB port, optical drive or the like.

The communication device 298 serves to permit and establish communication between the computer system 288 and one or more devices and/or computer systems, such as the transmitter electronics 36, which are external to the computer system 288. Exemplary communication devices 298 include a serial device, an Ethernet board or network connection, or the like.

The one or more power supply 299 is connectable to an AC and/or a DC source and serves to provide suitable power to the components of the computer system 288, such as the computer readable medium 294, the output device 296, the processor 292, the communication device 298 and the input device 297. Of course, some of the components of the of the computer system 288 may have their own independent power supply 299, and some may be shared. For example, if the computer system 288 is implemented as a tablet computer with an internal monitor (output device 296) and an external monitor (output device 296), then the internal and external monitors will typically have different power supplies 299. The power supplies can be implemented in a variety of manners such as a switching power supply, a battery or combinations thereof.

As will be discussed in more detail below, the pipe communication algorithm is stored on the computer readable medium 294 utilizing the input device 297 or the communication device 298 and installed onto the computer system 288 typically with the aid of an installation program running on the processor 292. In any event, once the pipe communication algorithm is installed onto the computer system 288, a priori information can be received by the processor 292 through the input device 297 and/or the communication device 298 and then analyzed with the pipe communication algorithm to generate the acoustic transfer function 290.

The computer system 288 can be implemented in a variety of manners such as a personal computer, a mainframe computer, a distributed processor computer system, a mainframe computer, a tablet computer, or the like. The pipe communication algorithm can be run by the computer system 288 had any suitable location, such as at a headquarters or research center, or at the location of the tubing 14.

Referring now to FIG. 9, shown therein is an exemplary section of the tubing 14 formed of multiple pipe sections 300 a, 300 b, 300 c, 300 d, 300 e and 300 f. The pipe sections 300 a-f are substantially identical in construction and function and so only one of the pipe sections 300 a will be described herein. In particular, the pipe section 300 a, for example, is provided with a box end 302 and a pin end 304. The box end 302 includes internal threads (not shown) and the pin end 304 includes external threads (not shown). The pipe sections 300 a-f are connected together by threading the external threads of the pin end 304 of one pipe section 300 a-f into the internal threads of the box end 302 of another pipe section 300 a-f. As used herein, a pipe length 306 is defined as a distance between the box ends 302 of adjacently disposed and connected pipe sections 300 a-f.

When two of the acoustic modems 25Mi+2 and 25Mi+1, for example, are located on the tubing 14, the first step to establish communication is to find a frequency of modulation which enables such communication. As discussed above, the receiver electronics 38 of the acoustic modems 25Mi+2, 25Mi+1, 25M, or 25Mi−1 can have an identical set (typically 10) of predefined carrier frequencies stored in each acoustic modem 25Mi+2, 25Mi+1, 25M, or 25Mi−1 to be used for synchronizing with the acoustic messages. Acoustic modem 25Mi+2, for example, first broadcasts the acoustic message at frequency #1. If the receiver electronics 38 of the acoustic modem 25Mi+1 detects such acoustic message, the transmitter electronics 36 of the acoustic modem 25Mi+1 preferably replies to the receiver electronics 38 of acoustic modem 25Mi+2 at the same frequency which is then chosen as the carrier frequency between the acoustic modems 25Mi+2 and 25Mi+1. If the acoustic modem 25Mi+1 does not detect the acoustic message, acoustic modem 25Mi+2 broadcasts another acoustic message at frequency #2, and so on, until acoustic modem 25Mi+1 answers.

If the set of predefined frequencies is randomly chosen, then the process of finding a possible frequency of communication can be lengthy, even between two acoustic modems 25Mi+1 and 25Mi+2, for example. In a typical downhole installation, between 10 to 20 acoustic modems 25 will be located on the tubing 14 and spaced between 1000-2000 ft. Establishing the full network between surface and bottom can then last up to hours, which can be detrimental to the efficiency of operations, especially if the network fails to operate in a given configuration: The full network has then to be rediscovered, with a noticeable interruption in data communication.

To reduce the time to establish the network, it is proposed to select the set of predefined frequencies, based on a priori knowledge available at the beginning of the operations. The a priori knowledge can include the following:

A pipe tally including information indicative of the pipe sections 300 to be used to create the tubing 14 including, for example, an average pipe length, and thus an estimation of the location of the band pass. A pipe length standard deviation, and thus an estimation of the attenuation associated with the band pass (because attenuation is frequency dependant, it favors the choice of lower frequencies). Knowledge of operations to be performed during the DST job: For example, replacing the water/brine originally present inside the pipes with lighter fluids such as gas or oil slightly reduces the frequency location of the band pass. Knowledge of an acoustic transducer response: If the transducer is resonant, then the frequency of operation should be biased around this resonance frequency. Experimental knowledge on the expected noise level: Flow induced noise is frequency dependant, with decreasing amplitude at higher frequencies. This effect could favor higher frequencies of operations.

This a priori information is input into the computer system 288 to produce an expected signal to noise ratio on the acoustic modem 25Mi+2, 25Mi+1, 25M, or 25Mi−1 versus frequency, and a list of expected preferred carrier frequencies based on this ratio, and to be written into each acoustic modem 25Mi+2, 25Mi+1, 25M, or 25Mi−1 before the start of the job. This list should preferably be ordered such that the first frequency has the highest probability of successful communication, again to minimize the time of network discovery.

The tubing 14 formed of pipe sections 300 a-f with equal pipe lengths 306 is a periodic medium. The pipe sections 300 a-f, for example, are connected together via the pin end 304 and the box end 302. The cross-section of the box end 302 is larger than the cross-section of the remainder of the pipe section 300 a. The acoustic message travelling along the tubing 14 is partially reflected at each cross section change and these periodic reflections generate a transfer function characterized by band pass zones 320 a-d (only four of which are labeled in FIG. 10 for purposes of clarity), and band stops 324 a-d (only four of which are labeled in FIG. 10 for purposes of clarity) in the frequency domain as shown in FIG. 10. For purposes of example, it is assumed that propagation at frequencies within the band pass 320 a-d is un attenuated and desirable for communication, while propagation at frequencies within a band stop 324 a-d is severely attenuated and must be avoided for successful communication over a significant distance. One of the main challenges of acoustic communication is selecting the proper frequency of communication for a reliable operation.

The center frequency (˜270 Hz) of the first band stop 324 a is such that the half wavelength λ/2 is equal to the pipe length 306 (˜10 m), and higher order band stops frequencies are multiples of this value. The frequency width of the band pass 320 a-d depends upon the order of the band stop 320 a-d, and decreases with the frequency in the range 0-5 kHz. The upper and lower frequency bounds of each band pass 320 a-d can be predicted from the knowledge of the pipe and connection characteristics (lengths and cross sections) and the acoustic propagation velocity in the tubing 14. A strong reflection coefficient at the pipe/connection boundaries, typical of a drill pipe, gives a rather narrow band pass.

The following is an exemplary equation for predicting the frequency location of the band stops:

g=cos(ωL₁ /c)cos(ωL ₂ /c)−0.5(S ₁ /S ₂ +S ₂ /S ₁)sin(ωL ₁ / c)sin(ωL ₂ /c)>1

Where: L₁ is the pipe length; L₂ is a length of the box end 302; S₁ is a cross section area of the pipe section; and S₂ is a cross-section area of the box end 302, c is a velocity of the acoustic signals propagating through steel, i.e., 5210 m/s, and ω is 2πf, where f is the frequency.

FIG. 12 shows the function g versus frequency, for a particular case with L₁ equal to 354 inches; L₂ equal to 16 inches, S₁ equal to 3.7 inches²; and S₂ equal to 8.2 inches². Shaded sections within FIG. 12 show locations of band stops 324 a, 324 b, 324 c, 324 d, 324 e, 324 f, and 324 g. For example, the function g is larger than 1 between 794 and 852 Hz, which can be identified as the band stop 324 c.

The intended acoustic mode of propagation is the extensional mode in the tubing 14, and its velocity is predominantly controlled by the steel material properties (bulk longitudinal and shear velocities and density), which are reasonably known and constant. However, the extensional velocity is also slightly affected by the fluids present inside and outside the tubing 14. The fluids (especially inside) are expected to change during DST operations: typically water/brine will be replaced by formation fluids (oil or gas), or by nitrogen pumped from surface in order to reduce the downhole pressure prior to open the testing valve. These fluid substitutions slightly changes the boundaries of the band pass 320 a-d, in a predictable way, once approximate values of their properties (acoustic velocity and density) are known.

Actual pipe sections 300 a-f are not strictly identical in lengths, and this is especially the case for drill pipe which can be repaired by cutting part of the connection to re-machine the threads. When the pipe lengths 306 of the pipe sections 300 a-f in the tubing 14 are not equal, then it can be shown that this introduces attenuation within the band pass 320 a-d as shown in FIG. 11. FIG. 11 show simulations for the attenuation across 30 pipe sections 300. Shown in the solid lines is the simulation with pipe sections 300 of identical length, and shown in the dotted lines is the simulation with pipe sections 300 of non-identical lengths, but with a standard deviation of 0.15 m. As shown in FIG. 11, the higher the standard deviation of pipe lengths 306, the stronger the attenuation as the carrier frequency and the overall effect is to restrict the range of possible communication frequencies to lower frequencies.

Although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of the present invention. Accordingly, such modifications are intended to be included within the scope of the present invention as defined in the claims. 

1. A method of transmitting data along tubing in a borehole, the method comprising the steps of: (i) transmitting an acoustic message containing data by a first modem at a first location on the tubing at a first frequency and a first bit rate selected from a predetermined group of at least two frequencies and at least two bit rates; (ii) receiving the acoustic message by a second modem at a second location on the tubing, and detecting the first frequency of the acoustic message by receiver electronics of the second modem; and (iii) synchronizing the acoustic message with the detected first frequency and in parallel with at least two bit rates by the receiver electronics of the second modem.
 2. The method of claim 1, wherein step (i) further comprises adjusting a frequency of the acoustic message to one of the at least two frequencies in the predetermined group of at least two frequencies and at least two bit rates.
 3. The method of claim 2, wherein the predetermined group of at least two frequencies and at least two bit rates comprises the first frequency and more than two further frequencies, the method further comprising iterating steps (i)-(iii) through the group of frequencies.
 4. The method of claim 1, wherein step (i) comprises adjusting the bit rate of the acoustic message to a lower bit rate.
 5. The method of claim 4, wherein the step of adjusting the bit rate follows adjustment of frequency of the acoustic message.
 6. The method as of claim 1, further comprising the step of re-transmitting the data within the acoustic message received by the second modem to a third modem.
 7. The method of claim 1, further comprising the step of determining whether the step of synchronization is successful utilizing a predetermined selection algorithm; and wherein the predetermined selection algorithm is selected from a group of at least one signal quality parameter consisting of signal distortion, signal strength, ambient noise, signal-to-interference noise ratio, signal-to-noise ratio, channel response time, signal amplitude, and signal auto-correlation.
 8. A method, comprising the steps of: converting an acoustic message into an electrical signal by a transceiver assembly of an acoustic modem attached to a tubing within a borehole, the acoustic message including a synchronization frame and transmitted at a first frequency and a first bit rate selected from a group of multiple frequencies and bit rates; receiving the electrical signal by receiver electronics of the acoustic modem; and synchronizing, in parallel by the receiver electronics of the acoustic modem, a synchronization frame of the electrical signal with at least two frequencies and at least two bit rates of the group of multiple frequencies and bit rates.
 9. The method of claim 8, wherein the multiple frequencies and bit rates within the group are predetermined.
 10. An acoustic modem for communication in a network of acoustic modems via a communication channel, the acoustic modem comprising: a transceiver assembly adapted to convert acoustic messages into electrical signals; transceiver electronics, comprising: transmitter electronics to cause the transceiver assembly to send acoustic signals into the communication channel; receiver electronics comprising at least one microcontroller adapted to execute instructions to (1) enable the receiver electronics to receive electrical signals indicative of the acoustic message from at least one other acoustic modem via the transceiver assembly, (2) estimate a carrier frequency of the electrical signals by analyzing an estimation frame of the electrical signals, (3) estimate a starting time of a data frame of the electrical signals by synchronizing with a synchronization frame of the acoustic message in parallel with at least two bit rates, and (4) decode the data frame; and a power supply supplying power to the transceiver assembly and the transceiver electronics.
 11. The acoustic modem of claim 10, wherein the at least one microcontroller of the receiver electronics enables the transmitter electronics to transmit an acknowledgement.
 12. The acoustic modem of claim 10, wherein the at least one microcontroller executes instructions to cause (2) and (3) to execute sequentially.
 13. An acoustic modem for communication in a network of acoustic modems via a communication channel, the acoustic modem comprising: a transceiver assembly adapted to convert electrical signals into acoustic messages; transceiver electronics, comprising: transmitter electronics coupled to the transceiver assembly; receiver electronics coupled to the transceiver assembly; at least one microcontroller executing instructions to (1) enable the transmitter electronics to enter a training phase where the transmitter electronics transmit a training message having an estimation frame, a synchronization frame and a data frame to the transceiver assembly, and (2) enable the transmitter electronics to enter a data communication phase where the transmitter electronics transmits a data message to the transceiver assembly having a synchronization frame and a data frame without an estimation frame; and a power supply supplying power to the transceiver assembly and the transceiver electronics.
 14. An acoustic modem for communication in a network of acoustic modems via a communication channel, the acoustic modem comprising: a transceiver assembly adapted to receive an acoustic message having a synchronization frame and a data frame and to convert the acoustic message into an electrical signal; transceiver electronics, comprising: transmitter electronics to cause the transceiver assembly to send acoustic signals into the communication channel; receiver electronics having at least one microcontroller executing instructions to (1) receive the electrical signal indicative of the acoustic message; (2) synchronize, in parallel, a synchronization frame of the electrical signal with at least two frequencies and at least two bit rates of a group of multiple frequencies and bit rates; and (3) decode the data frame; and a power supply supplying power to the transceiver assembly and the transceiver electronics.
 15. An acoustic modem for communication in a network of acoustic modems via a communication channel, the acoustic modem comprising: a transceiver assembly adapted to convert an electrical signal into an acoustic message; transceiver electronics, comprising: receiver electronics coupled to the transceiver assembly; transmitter electronics coupled to the transceiver assembly and including at least one microcontroller adapted to execute instructions to generate a sequence of electrical signals directed to the transceiver assembly with different carrier frequencies and with the sequence ordered based upon a model of a particular, planned tubing for a borehole; and a power supply supplying power to the transceiver assembly and the transceiver electronics.
 16. A computer system, comprising: one or more input device; one or more output device; one or more non-transitory computer readable medium storing instructions for (1) receiving a priori information of a particular, planned tubing for a borehole from the one or more input device, (2) predicting optimal carrier frequencies for acoustic communication utilizing the particular, planned tubing as a communication channel between two or more modems, and (3) outputting information to the output device indicative of the optimal carrier frequencies; at least one processor in communication with the one or more non-transitory computer readable medium for executing the instructions based upon a message from the one or more input device; and one or more power supply supplying power to the input device, the output device, the one or more non-transitory computer readable medium and the one or more processor. 